Process for making pulp and paper



United States Patent 2,731,344 PROCESS FOR MAKING PUL AND PAPER Ralph H. McKee, New York, N. Y.

No Drawing. Application May 11, 1953, Serial No, 354,397

5 Claims. (Cl. 92-9) This invention relates to the production of fiber pulp and of paper from lignin containing cellulosic materials such wood, straw, flax, bagasse, jute and the like; it is a continuation-in-part of my pending application Serial No. 738,226, filed March 29, 1947, and now abandoned.

In conformity with my prior Patent No. 2,308,564 lignin containing cellulosic materials are digested a t an elevated temperature in a solution of one or a mixture of certain hydrotropic salts, such as, for instance, alkali or alkaline earth salts of benzoic, salicylic, xylenesulfonic, cymensulfonic, phenolsulfonic and toluenesulfonic acids. The digestive action does not dissolve the cellulose but it dissolves the lignin and simultaneously sets free carbon dioxide. At the conclusion of the digestion or cooking operation the cellulose is suspended as a pulp in the accompanying liquor which contains the lignin in solution.

It is an object of this invention to produce a cellulose pulp carrying a maximum of the weight of the processed materials; if wood is used as an initial charge the yield of the pulp may reach 75 per cent.

It is a further important object of the invention to produce a water insoluble pulp consisting primarily of cellulose and lignin which is surprisingly Well adapted for its conversion into paper of a high rigidity.

it is another object of the invention to produce an unsized paper pulp which by the customary drying and hot calendering steps of paper mill practice is converted into a self-sized paper.

It is also an object of the invention to permit the use of an additional sizing step, such as for instance rosin sizing.

A further important object of the invention is to utilize the cellulose pulp obtained in conformity therewithfor the production of heavy, stifi papers and particularly wrapping papers and paper boards having a thickness of, for instance, 9 to 16 thousandths of an inch, these papers being particularly well adapted for the manufacture of corrugated papers and liners for corrugated cardboard boxes.

It is also an object of the invention to impart to an unbleached paper or paper board a surprisingly high degree of rigidity or stiffness. This rigidity is particularly useful for making a strong corrugated paper.

It is a further object of the invention to confer upon paper and paper board a high finish, a dark color and "to increase its resistance to water penetration.

With the above recited and additional objects in view which will become apparent as this specification proceeds, the invention will now be described in the following.

Cellulosic lignin containing raw materials, such as wood, straw, bagass es, flax straw, jute and the like are heated and digested with a hydrotropic solution such as described in my Patent No. 2,308,564 to produce a crude fiber pulp. 1

ln' deviation from the teachingsof rnyprior invention, the lignin solution 'is notor only partially separated from the resulting cellulose fiber pulp. v

Moreover, the pulp suspension is kept in acold slightly 2,731,344 Patented Jan. 17, 1956 2 acid state while the lignin contained in the solution and in the cellulose fibers is precipitated in a gelatinous form from the same by the admixture of cold water or cold dilute aqueous solutions directly on and in the cellulose fiber.

The lignin thus precipitated from the solution is largely in a gelatinous form which is quite dilferent from the granular and highly polymerized form which is obtained when hot lignin solutions or suspensions of cellulose in a hot hydrotropic solvent are mixed with hot water in accordance with the procedure of my prior Patent No. 2,308,564. The gelatinous type of lignin as obtained by the cold precipitation, which, as such, is diificult to filter from the dilute hydrotropic salt solution, can be easily filtered from the solution in the presence of the pulp fibers. This lignin which is light brown in color will be denoted in the following as cold hydrotropic lignin. it difiers from commonly known types of lignins such as soda lignin, sulfite lignin, phenol lignin, sulfuric acid lignin, etc. in color, solubility, fusion temperature and by th fact that it is much less polymerized than the customary approximately black lignins.

The precipitation of the lignin on and in the fibers is accomplished by mixing with the cellulose pulp water at approximately room temperature which is denoted as cold water in contradistinction to the usual hot precipitation of the lignin taking place at about C. in a fiocnlent form which becomes granular in a few minutes and filterable by mixing the lignin solution with hot water, preferably under conditions wherein the mixture has a pH on the acid side.

The method may be modified by separating, after digestion but before precipitation, a portion of the lignin containing liquor from the fiber pulp, for instance, by filtering or pressing and then precipitating the lignin from the remaining solution onto the fibers with reuse of the strong filtrate for the digestion of a fresh batch of cellulosic raw materials. The dilute liquor which results from the precipitation washing of the pulp may be concentrated by evaporation to a digesting concentration of preferably about 30 per cent of hydrotropic salt content and then reused for the digestion of a fresh batch of cellulosic materials. The process may be modified by only partially digesting the cellulosic charge with the hydrotropic agent and then completing the pulping by a mechanical. treatment, for instance, by a disintegration of the same in a mechanical equipment such as disk disintegrators and the like.

The hydrotropic lignin cold precipitated on and in the cellulose fibers in accordance with the invention is water insoluble and in part readily fusible at a temperature usable for thehot calendering of paper; a temperature corresponding to steam of about 50 pounds gauge pres sure suffices for this purpose. The required exact temperature will depend on the amount and the composition of the lignin. About one-third of the lignin dissolved by the hydrotropic solvent is of the fusible type, but the proportion 0f fusible lignin precipitated on the pulp fibers will vary with the method use to bring about the lignin precipitated and the completeness of the precipitation.

If paper is made from the hydrotropic lignin containing fiber pulp wherein calendar temperatures are reached which fuse the lignin, the paper will be automatically selfsized by the fused lignin; the thus sized paper has practically the properties of a paper which is impregnated with a commercial thermoplastic resin; it will require less or no additional sizing with rosin and alum, which are the hitherto customary sizing agents.

Before being converted into paper the pulp is beaten whereby the strength of the paper sheets is increased. This refining step may be carried-out either before removal and return of the excess of the cooking solution for of the lignin and the subsequent reuse in the digester or after the precipitation by cold water of the slightly polymerized lignin in and on the fibers. It is preferable to refine before removal of the excessofthe cooking .solvent as attthat time the refining can becarried, out in, ahot state and therebythe time required for refining and the power requirement is reduced.

The refined pulp is then screened on a wire screen to remove. the hydrotropic salt solution and the bulk of the suspendedlignin particles not'attached to the cellulose fibers. This solution mayhave its suspended lignin removed by filtration by use of a rotary or other filter, before. being brought to the evaporators for concentration to the desired strength for reuse in the digester. It is not necessary, but desirable, to remove the suspended lignin before concentration in the evaporators as on concentration the suspended lignin will redissolve in the resulting solution.

The refined pulp may now be converted by customary methods into a nine point paper board having a thickness of 0.009 inch of the type used for corrugation. The resulting nine point paper was stronger by the Mullen test by fully forty per cent than similar paper made by hot,

. for instance at 95 C., precipitation of the lignin, both papers being made in the same way without the addition of any rosin size or other foreign material and both dried at a temperature of about 60 C.

The paper thus produced was hot calendered at a temperature which softened the more fusible constituent of the lignin; the thickness was reduced by about eight to ten per cent; it also was somewhat darker in color, had a nearly 100 per cent higher rigidity (stiffness) and gave, as might be expected from the increase in rigidity, a lower Mullen pop test by about twenty-five per cent than the same paper which had not been hot calendered. The high rigidity renders a sheet of the stiff paper particularly desirable for the making of corrugated sheets and supported in the customary way by attached liners it has the high crushing strength for corrugated box boards required by the container box industry.

By the use of heated corrugated rolls the hot calendering and corrugations can be simultaneously efiected and the corrugations be set by simple air cooling. Therefore, the expensive final drying step, normally used when paper is steam or water softened, then corrugated and then redried can be omitted; nevertheless, a stronger and much more rigid corrugated sheet is obtained.

If instead of cold Water for the precipitation of the lignin, dilute aqueous solutions of hydrotropic salts such as the pulp wash solutions are used, the load to be handled by the evaporators will be decreased.

Tests as to the water penetration of the papers showed that before hot calendering a drop of water placed on the sheet and dried at 60 C. penetrated through the sheet in about five seconds; after hot calendering and fusing of part of the lignin the time required for a drop of water to penetrate through the sheet was two to four thousand times longer, depending on the completeness of the fusion of the lignin.

When the lignin is hot precipitated on the fibers, the resulting pulp and paper are darker in color; the resulting paper made in the same way as previously described and likewise dried at 60 C. gave before'hot calendering a water penetration time of about five seconds and after hot calendcring only four to six times as long, also the rigidity had not increased measurably; in other words, the lignin was more polymerized and did not well size by fusing.

Moreover, addition of other types of lignin in powder forms such as soda lignin, kraft lignin, sulfite lignin and sulfuric acid lignin always decreased thestrength of the paper, its rigidity and its resistance to water penetration whether cold calendered or hot calendered. These lignins all acted as highly polymerized inert materials in contrast to the light brown cold-precipitated fusible hydrotropic lignin.

If the presence of a rosin size, plastic, dye or other additional material is intended to appear in the paper, it may be added to the pulp before it is converted into paper. However, with this lignin sized paper a less than customary amount of the additive material will be required. V v

Inthe cold pulp making process, which has just been described, carbon dioxide gas will be released from the pulp during the cooking operation. I prefer to carry out the process so that the gauge pressure stays in the range of to pounds per square inch. The relief of this gas can be either continuous or intermittant relief, but the preference is for setting a valve so that the relief is of a continuous type. The removal of the carbon dioxide gas evolved does not decrease the efiiciency of the cooking process as the removal of sulfur dioxide does in the case of the sulfite pulping process.

The invention is well adapted to be used in the batch process such as described in my prior Patent No. 2,308,564 or in my pending patent application for a continuous process, Serial No. 228,476 filed May 26, 1951. Economically the advantage is with the continuous process in that the time of digestion is shorter, the temperatures used average somewhat higher and the ratio of solvent to cellulosic raw material is under better control. The steam requirement is much less and the labor requirement is also materially less than for the batch process.

The invention will now be described with reference to the following specific example.

Air-dried sugar cane bagasse which had been cut into pieces of maximum length of two inches was heated in a digester with seven times its weight of a 33 per cent solution of sodium xylenesulfonate at a temperature of about 168 C. for about one and a half hour with a release of the evolved carbon dioxide such that the attached pressure gauge registered 90 lbs. during the digestive cooking operation.

The resulting pulp suspension was removed from the digester and refined in a rotating ball mill having a small charge of pebbles for 20 minutes at 40 R. l. M. Pulp and solvent were separated from the pebbles.

The resulting defibered pulp wet with the cooking solution now had a temperature of about 50 C. It was mixed with twice its volume of cold water of between zero and-50 C. and preferably about 15 C. and the whole stirred for ten minutes and then filtered through a wire screen. The filtrate was a solution of about 10 per cent sodium xylenesulfonate and still carried in solution sufficient lignin to have a coffee color. The pulp was washed with cold water to remove the cooking solution and unattached particles of lignin and examined. This pulp when dry was of light brown color and under the microscope when wet appeared to have light brown lignin precipitated in and on the pulp fibers. This is the characteristic appearance of the slightly polymerized gelatinous lignin obtained when hydrotropic lignin is precipitated by dilution with cold water of a nearly neutral solution of lignin in a hydrotropic solvent.

This pulp was converted by customary hand sheet methods into a nine point (0.009 inch thick) paper board of the type used for corrogation. No addition was made of rosin size or other extraneous material. For comparison exactly similar sheets of bagasse pulp paper were made. from pulp whose lignin had been precipitated by hot C.) precipitation. Both sheets were dried at 60 C. The sheets made by.cold precipitation of the lignin showed on the average a 40 per cent higher Mullen strength test.

The paper made from the pulp produced from the cold precipitation was hot calendered at a temperature of about 160 C. which softened the more fusible constituent of the lignin; the resulting paper was now thinner by about 8 to ,9 per cent, somewhat darker in color, had nearly per cent higher rigidity (stiffness); it gave, as might be expected from the increase in rigidity, a lower Mullen pop test by about twenty-five per cent that the same paper which had not been hot calendered. This high rigidity makes a sheet of stiff paper of a type particularly desirable for a corrugated sheet, supported in the customary way by attached liners, as it gives the high crushing strength to the corrugated box boards needed by the container box industry.

Tests as to the water penetration of the new papers showed that before hot calendering a drop of water placed on the sheet dried at 60 C. penetrated through the sheet in about five seconds of time but after hot calendering (fusing of part of the lignin) another part of the same sheet the time required for a drop of water to penetrate through the sheet was more than four thousand times longer.

When the lignin was precipitated hot (95) on the fibers, the resulting pulp and paper were darker in color and the resulting paper made in the same way as previously described and likewise dried at 60 C. gave before hot calendering a water penetration time of about five seconds and after hot calcndering only four to six times as long; also the rigidity had not increased measurably. In other words, the hot precipitated lignin was more polymerized and did not size by fusing.

Comparison with the corrugated paper and the two liners of commercial corrugated boxes showed the corrugated sheets to be nine to twelve point, to have a water penetration of five seconds or less, and to have a rigidity due to sizing of below 60 per cent of my hot calendering nine to ten point darker sheet. The two liners were nine to sixteen point in thickness, fairly rigid papers with a water penetration time of always less than twenty seconds. This in contrast with a time of four to six hours used for my hot calendered sheet.

Since certain changes in carrying out the above process .could be made without departing from the scope thereof,

it is intended that all matters contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What I claim is:

1. In the process of making a self-sizing pulp for use in the manufacture of paper from cellulosic material the improvement which comprises suspending cellulosic raw materials in a column of a hot hydrotropic digesting liquor, controlling the temperature of said liquor to efiect digestion of said materials while in suspension, withdrawing the resulting pulp suspended in the liquor containing the dissolved lignin, cooling said suspension of pulp and liquor to approximately room temperature, diluting said suspension with about twice its volume of cold water having a temperature of about C., thereby precipitating cold hydrotropic lignin in and on the pulp fibers, washing the impure lignin pulp,concentrating the washliquors to a concentration of about 30 per cent of the hydrotropic salt and re-using said concentrated liquor in the pulp digestion process.

2. A process of manufacturing paper comprising hot digesting a lignin containing cellulosic material with a hydrotropic salt solution to produce in said hydrotropic lignin solution a suspension of unbleached pulp, cooling said pulp suspension, adding cold water having a temperature of about 15 C. and approximately twice the volume of the pulp suspension whereby lignin is precipitated in and on the fibers, producing from this lignin containing pulp a paper sheet, drying said sheet, and heating it to a temperature to fuse a portion of the lignin and thereby simultaneously doubling the rigidity of the paper and sizing the paper to resist a water penetration of about four hours.

3. A process of manufacturing paper comprising cutting air-dried bagasse into pieces of a maximum length of about two inches, heating the same in a digester with about seven times its weight of a 33 per cent solution of sodium xylenesulfonate at a temperature: of about 168 C. for about one and a half hours, removing the resulting pulp suspension from said digester, refining it by frictional action, mixing it with about twice its volume of cold water having a temperature of about 15 C., stirring for about ten minutes, filtering the pulp, washing it with cold water and converting the said pulp into paper.

4. A process for making a paper pulp comprising hot digesting a lignin containing cellulosic material with a hydrotropic salt solution to produce a suspension of cellulosic fiber pulp and a substantially neutral hydrotropic lignin solution, removing a portion of the hydrotropic lignin containing liquor, cooling the remaining portion of the suspension, mixing the pulp suspension with cold water having a temperature of about 15 C., thereby precipitating the hydrotropic lignin from its solution in and on the fibers and reusing the said removed liquor portion for the digestion of an additional amount of cellulosic raw materials.

5. The process for making a paper pulp comprising heat-digesting a lignin containing cellulosic material with a hydrotropic salt solution, producing thereby a suspension of the crude cellulosic fiber pulp in said solution by keeping the pulp and the hydrotropic solution in mutual contact, cooling the pulp and the lignin solution and precipitating the hydrotropic lignin from its solution directly in and on the pulp by dilution of the same with cold water having a temperature of about 15 C.

References Cited in the file of this patent UNITED STATES PATENTS 23,099 Lowe Mar. 1, 1859 1,716,623 Collins June 11, 1929 1,761,069 Booth June 3, 1930 1,873,056 Smith et al. Aug. 23, 1932 2,056,810 Smyser Oct. 6, 1936 2,308,564 McKee Jan. 19, 1943 2,338,602 Schur Jan. 4, 1944 FOREIGN PATENTS 482,894 Great Britain Apr. 6, 1938 461,373 Canada Nov. 29, 1949 OTHER REFERENCES Lau: Paper Ind. and Paper World, June 1941, pp. 249- 253.

Hydrotropic Solutions as Solvents of Lignin, by Pelipetz (1937), pp. 22, 23, 28, 29, 33, and 34.

McKee: Ind. and Engineering Chem, April 1946, p. 384. 

5. THE PROCESS FOR MAKING A PAPER PULP COMPRISING HEAT-DIGESTING A LIGNIN CONTAINING CELLULOSIC MATERIAL WITH A HYDROTROPIC SALT SOLUTION, PRODUCING THEREBY A SUSPENSION OF THE CRUDE CELLULOSIC FIBER PULP IN SAID SOLUTION BY KEEPING THE PULP AND THE HYDROTROPIC SOLUTION IN MUTUAL CONTACT, COOLING THE PULP AND THE LIGNIN SOLUTION AND PRECIPITATING THE HYDROTROPIC LIGNIN FROM ITS SOLUTION DIRECTLY IN AND ON THE PULP BY DILUTION OF THE SAME WITH COLD WATER HAVING A TEMPERATURE OF ABOUT 15* C. 